With such infrared sensor assemblies that are used for motion detectors and noncontact temperature measurement (pyrometry), for example, optical elements (e.g., lenses, mirrors, apertures and the like) are often provided to restrict the field of view of the infrared sensor. Likewise, protective planar windows or films are used to protect mirror lenses from environmental influences in particular.
A typical infrared sensor assembly for pyrometry comprises an IR sensor element (e.g., a thermopile radiation sensor, a pyroelectric radiation sensor, a bolometer and the like), optical elements for focusing (lenses, mirrors, etc.), apertures, filters, etc. as well as a housing, optionally in multiple parts, in which the various elements are combined as part of same.
Typical of this type of sensor is a TO housing 1 having a base plate 2 (also known as a “header”) and a cap 3, in which a sensor element 4, for example, a thermopile chip, usually a reference sensor 5 for measuring the housing temperature and an optical element 6, e.g., a lens, an aperture and/or a planar window are provided (see FIG. 2). The lens and the IR sensor element define a field of view 7 of the sensor according to the laws of radiation optics. Rays 8 within this field of view reach the sensor element 4 by simple transmission. Rays 9a outside of the intended field of view do not reach the sensor element 4 by simple transmission. Nevertheless, rays 9b outside of the intended field of view can reach the sensor element 4 by multiple reflections inside the optical element 6 and may thus distort the measurements.
An IR sensor assembly with or without a lens is described in U.S. Pat. No. 4,626,686, wherein a thermopile radiation sensor is mounted from the rear in a tube, the rear side of the sensor housing not being attached to the tube. An aperture is mounted first in front of the radiation sensor (opposite the radiation incidence) with a lens and/or a planar window mounted behind the aperture. The aperture is thus placed between the sensor element and the lens and/or window, with everything together mounted in a tube, optionally comprising multiple parts.
Such sensor assemblies have various disadvantages:
IR-transparent materials often used for lenses and windows have relatively high refractive indices n (e.g., silicon n≈3.46, germanium n≈4.0, polyethylene n≈1.7), which may produce strong reflections on the interfaces. Incident IR radiation may be reflected repeatedly inside the optical element under some circumstances. Through such multiple reflections, IR radiation, which is actually outside of the calculated field of view, may reach the sensor element. This effect may also occur with mirror optics equipped with a protective window. Multiple reflections then do not occur inside the optical element itself but may occur within the combination of a mirror and a protective window.
The thermal behavior of the optical elements of the infrared sensor assembly differs from that of the actual sensor element (e.g., thermopile chip), i.e., the optical elements heat up (cool down) in relation to the actual sensor element. This results in additional thermal effects, which can also distort measurements. Uneven heating (or cooling) of the optical elements may be caused by thermal conduction and/or convection of the ambient media or by thermal radiation of the object to be measured.
The housing itself of the actual sensor (e.g., a thermopile chip in the TO-5 housing) may be heated (cooled) unevenly. The causes (thermal conduction, convection, thermal radiation) may result in thermal effects (e.g., additional IR radiation) which can distort the measurement.
U.S. Pat. No. 4,797,840 describes another known sensor assembly, wherein a tube is arranged in front of a pyroelectric sensor to restrict the field of view. The tube has a mirror plating on the inside to prevent the tube itself from heating up and then emitting IR radiation. This reduces one of the disadvantages, but mirror plating can cause multiple reflections, so that unwanted radiation from almost any angle (outside of the field of view) may reach the sensor element.
U.S. Pat. No. 5,018,872 describes how a sensor housing of an IR sensor assembly can be designed to be more thermally homogeneous by means of a heat exchange body (“heat sink”). The field of view is determined by means of an aperture and a safety window. Multiple reflections cannot be prevented completely.
WO 201004505 describes a radiation trap connected thermally to the sensor housing. However this trap is situated between the lens and/or mirror and the sensor element. This reduces the problem of uneven heating of the housing but does not eliminate the other disadvantages described above.
DE 10 2004 030 418 discloses the design of a sensor housing in which a few parts are bonded to the sensor chip on a wafer level. This composite is then inserted into a plastic or ceramic housing, where all the parts manufactured in the wafer composite are effectively thermally coupled (thermally homogeneously). However, the external aperture is only incompletely coupled to the sensor chip both thermally and structurally as well as through the choice of plastic or ceramic as the material. Distorting thermal effects may thus occur. The optics of the wafer composite, including the external aperture, cannot completely prevent multiple reflections.
Improved infrared sensors and sensor assemblies are desired.